Androgen-binding Protein/sex Hormone-binding Globulin Research Articles

The rat androgen-binding protein (ABP/SHBG) gene contains triplet repeats similar to unstable triplets: Evidence that the ABP/SHBG and the fragile X-related 2 genes overlap

The rat androgen-binding protein/sex hormone-binding globulin (ABP/SHBG) gene is regulated by promoters P1 and PA. P1 regulates the mRNA encoding secreted ABP/SHBG, whereas PA regulates an alternate mRNA which encodes a modified protein that is targeted to the nucleus. Promoter PA is GC rich, consisting of 70–80% GC residues. During routine BLAST sequence analysis it was discovered that this GC-rich region is highly related to the human fragile X-related protein 2 (FXR-2) 5′-untranslated RNA sequence. Furthermore, the nucleotide coding sequence of the initial 14 FXR-2 amino acid residues was identical in the ABP/SHBG gene. The 5′-untranslated FXR-2 sequence contains triplet (CGG) repeats, which are also present in the rat ABP/SHBG gene. The meiotic instability of CGG repeats in the human fragile X (FMR1) gene causes the fragile X mental retardation syndrome. The data presented here suggest that the ABP/SHBG and FXR-2 genes overlap with each gene transcribed in the opposite direction. In support of this structure, the human ABP/SHBG and the FXR-2 genes map to the same site on chromosome 17. Thus, the ABP/SHBG gene contains triplet repeats in the alternate promoter PA. It will be of particular interest to determine if triplet instability affects ABP/SHBG gene expression. A triplet instability in the X-linked androgen receptor gene causes spinal and bulbar muscular atrophy.

Overexpression of androgen-binding protein/sex hormone-binding globulin in male transgenic mice: tissue distribution and phenotypic disorders.

The rat androgen-binding protein/sex hormone-binding globulin (ABP/SHBG) gene in transgenic mice was previously shown to be specifically expressed in the testes. This study verifies a Sertoli cell location of ABP and translation of testicular ABP mRNA in the transgenic mice by dihydrotestosterone (DHT)-binding assays and immunohistochemistry. DHT-binding activities in the testis and epididymis of the hemizygous transgenic mice were elevated 20-fold as compared to activity in the wild-type tissues. DHT-binding activities were also elevated in blood plasma at least 25- to 50-fold in the transgenic mice; binding was undetectable in the plasma from control mice. Immunohistochemical analysis revealed that the transgenic testicular ABP was primarily in the cytoplasm of Sertoli cells and lumen of the seminiferous tubules. In some tubules, intense staining also was associated with spermatids. After transport to the epididymis, there were large amounts of immunoreactive ABP internalized in the epithelium of the initial segment and proximal caput. The increased levels of plasma and testicular ABP had no effect on levels of testosterone; there was a 30-fold range of plasma and testicular testosterone levels in the wild-type and transgenic mice. Increased ABP levels in the transgenic mice were associated with structural and functional abnormalities in the testis. Abnormal spermatogenesis resulted in extensive structural changes in the transgenic testis; the degree of the defect varied from near normality to the loss of most germ cells. In the affected mice, seminiferous tubules had smaller diameters and decreased numbers of germ cells, particularly in the spermatid stages of differentiation. Pyknotic nuclei and multinucleated cells were associated with the spermatids in the defective tubules, but not in the wild-type tubules. Consequently, mice with the spermatogenic disorder had reduced epididymal sperm numbers. The variable spermatogenic disorder was associated with variable male fertility. The homozygous transgenic male and female mice also had a serious motor dysfunction affecting their hind limbs. This study demonstrates how the transgenic mouse model can be used to study ABP's function, and the data support several hypotheses on its function in the testis and epididymis.

The alternate N-terminal sequence of rat androgen-binding protein/sex hormone-binding globulin contains a nuclear targeting signal.

Androgen-binding protein/sex hormone-binding globulin (ABP/SHBG) is an extracellular carrier protein of androgens and estrogens. The protein regulates the bioavailability of sex steroids, and expanding evidence suggests that it also acts as a hormone. ABP/SHBG is secreted by Sertoli cells and hepatocytes using a signal peptide. An alternate messenger RNA encodes a nonsecreted form of ABP/SHBG (Alt-ABP/SHBG) that has a unique N-terminal amino acid sequence. In this study, we report that the alternate N-terminal sequence targets Alt-ABP/SHBG to the nucleus instead of the endoplasmic reticulum. The recombinant Alt-ABP/SHBG expressed in COS-7 cells was located in the nucleus, whereas recombinant cellular ABP/SHBG was primarily cytoplasmic. Neither dihydrotestosterone nor estradiol had any detectable effect on the ABP/SHBG or Alt-ABP/SHBG cellular location. Although the function of the nuclear Alt-ABP/SHBG is unknown, it may act as a modulator of androgen receptor- and/or estrogen receptor-mediated gene regulation.

Genetic mapping of the gene for androgen-binding protein/sex hormone-binding globulin to mouse Chromosome 11

Southern blot analysis of DNAs from Chinese hamster x mouse and rat x mouse somatic cell hybrids showed that the mouse gene encoding androgen-binding protein/sex hormone-binding globulin (ABP-SHBG) is on Chromosome 11. Progeny from an intersubspecies backcross were analyzed to position this locus, termed Shbg, between Il-3 and Int-4 in the middle of this chromosome. Shbg is thus closely linked to several neurological mutations, one of which, Tr, is also associated with male sterility. The recent finding that ABP-SHBG is found throughout the rat brain raises the possibility that one of these mutations may be due to a defect in Shbg.